CORRECTING VLBI OBSERVATIONS FOR IONOSPHERIC REFRACTION

Radio astronomical observations with local interferometers and very lo ng baseline interferometers (VLBI) are affected by atmospheric (i.e. t ropospheric and ionospheric) refraction. A procedure to correct VLBI o bservations for ionospheric refraction using simultaneous ionospheric measurements is outlined. If networks of ionosonde and satellite obser ving stations operate during the astronomical VLBI observations, pathl ength corrections ranging from several times the VLBI observing wavele ngths to a fraction of this wavelength using the simultaneously collec ted ionospheric data are possible, provided that the geographical dist ribution of these ionosphere stations is similar to the VLBI network.